High viscosity elastomers are often used in tire tread rubber compositions. Such elastomers have a very high Mooney (ML4) viscosity value in their unvulcanized state in a range of, for example, about 70 to about 140. Elastomers of such high viscosity are typically difficult to process for preparation of rubber compositions because it is very difficult to mix rubber compounding ingredients such as, for example, carbon black and silica reinforcing fillers, in high viscosity rubber. This problematic aspect is well known to those having skill in such art.
In order to facilitate the processing of a high Mooney viscosity for rubber compositions such as tire tread stocks, namely the mixing and extrusion of the rubber composition, it is somewhat customary to use such elastomer as a pre-blend with a rubber processing oil to reduce its Mooney (ML4) viscosity to a more processable level such as, for example, in a range of about 45 to about 55. Such technique is well known to those having skill in such art.
However, it is considered herein that such use of a rubber processing oil can be disadvantageous because such oil may be substantially saturated hydrocarbon based and, thus, not participate in the vulcanization of the rubber composition. It thereby becomes somewhat of a diluent for the rubber composition and its resulting physical properties.
Liquid elastomers have also sometimes been used for such purpose with similar difficulties where the elastomers are substantially saturated in nature.
In the description of this invention, the term "phr" where used herein, and according to conventional practice, refers to "parts of a respective material per 100 parts by weight of rubber". Usually parts and percentages, where used herein, are in terms of weight unless otherwise noted.
A latex, insofar as this invention is concerned, is normally prepared by polymerizing diene monomers, optionally also with styrene,in a water-based medium, usually as an emulsion accompanied with suitable emulsifiers, with a free radical catalyst activators(s). Often a redox polymerization system is used. Descriptions of various polymerizations may be found, for example, in U.S. Pat. No. 3,080,334; in Synthetic Rubber by G. S. Whitby, 1954, particularly Chapter 8 and Emulsion Polymerization by F. A. Bovey et al, Vol. IX of "High Polymers", Interscience Publishers, Inc., 1955. Various organic initiators are described by J. Brandrup et al, Polymer Handbook (John Wiley & sons), 1965, pages II-3 to II-51. The resulting latex is then coagulated to recover the elastomer. Standard coagulation techniques, such as salt-acid coagulation procedures, can be employed. For example, a styrene/butadiene copolymer elastomer can be prepared by such aqueous emulsion polymerization process. The Mooney viscosity of the resulting copolymer can be controlled to some degree by the various polymerization parameters so that such copolymer of a relatively high Mooney or of a relatively very low Mooney (ML4) value can be prepared. It is believed that such aqueous emulsion polymerization processes are well known to those having skill-in such art.
By the term "polymerizate", "cement" or "polymerizate cement" where used herein, it is meant that it is the product of individually polymerizing monomers selected from at least one conjugated diene or at least one conjugated diene with an aromatic vinyl compound such as, for example, styrene and/or alpha-methylstyrene, in the presence of catalyst initiator(s) and in an organic solvent and stopping the actual polymerizing of the monomers but before the resulting polymer is removed from the polymerizate.
The terms such as "compounded rubber", "rubber compound" and "rubber composition" relate generally to "rubber which has been mixed with various rubber compounding ingredients". Such terms are well known to those skilled in the rubber mixing art, especially for tires.
The terms "vulcanized", "vulcanize", "cured" and "cure", if used herein, are used interchangeably to refer to the "vulcanization of rubber" and such terms are well known to those having a skill in the rubber vulcanization art.
The term "Tg", if used herein, refers to the "glass transition temperature of a particular elastomer". Glass transition temperatures are well known characterizations of elastomers. They may, for example, be suitably determined by a differential scanning calorimeter (DSC) instrument with a heating rate of 20.degree. C. per minute.
The term "Mooney viscosity" where used herein, unless otherwise specified, may be referred to as an (ML4) viscosity and refers to "a viscosity of an elastomer in its uncured state, and without appreciable additives dispersed therein other than antidegradants, measured by (or according to) ASTM Test Method D1646 conducted at 100.degree. C". Sometimes the test is referred to as ML1+4, a shorthand for meaning Mooney Large (using the large rotor) with a one minute static warm-up before determining the viscosity after four minutes. As used herein, a ML4 viscosity measurement is intended to mean the ML1+4 viscosity measurement.